As is the case for most meats, port grading also involves the principle of identification of economically important differences in quality and quantity.
In general, the standards provide two levels of quality: one for carcasses with acceptable lean quality and the other for carcasses with unacceptable lean quality. The quality of the lean can be evaluated by observing the cut surface of a major muscle, and when such a surface is available it can be used as the basis for the quality evaluation. The quality-indicating characteristics include such features as marbling, firmness and color. The degree of external fatness, as such, is not considered in evaluating lean quality.
In evaluating quality on a lean cut surface, some standards describe the characteristics of the loin-eye muscle at the 10th rib. However, when this surface is not available, other exposed major muscle surfaces, such as the ends of the loin, are used. Such evaluations are based on the normal development of the quality-indicating characteristics in relation to their development in the loin-eye muscle at the 10th rib.
When a major muscle cut surface is not available, the quality of the lean is evaluated indirectly, based on quality-indicating characteristics that are evident in carcasses. These include firmness of the fat and lean, amount of feathering between the ribs, and color of the lean.
Carcasses which have unacceptable lean quality or bellies too thin to be suitable for bacon production are normally graded U.S. Utility. Soft and oily carcasses are also normally graded U.S. Utility regardless of their development of other quality-indicating characteristics.
Carcasses with acceptable lean quality and acceptable belly thickness are graded U.S. No. 1, U.S. No. 2, U.S. No. 3, or U.S. No. 4. These grades are based almost entirely on the expected carcass yields of the four lean cuts, i.e. ham, loin, picinic shoulder, and Boston butt. The expected yields of the four lean cuts for each of these four grades have been tabulated as follows:
______________________________________ Expected Yields of the Four Lean Cuts Based on Chilled Carcass Weight ______________________________________ Grade Yield ______________________________________ U.S. No. 1 53 percent and over U.S. No. 2 50 to 52.9 percent U.S. No. 3 47 to 49.9 percent U.S. No. 4 Less than 47 percent ______________________________________
It can be expected that these yields will be approximately 1 percent lower if based on hot carcass weight.
These yields are based on the normal cutting and trimming methods used by the USDA in developing the standards. In general, this involves closer trimming of the fat than is usual in commercial practice. Different yields may result from other methods of cutting or trimming. However, if these methods are applied uniformly, differences in yields between the grades will remain relatively comparable.
Carcasses differ in their yields of the four lean cuts because of differences in their degrees of fatness and in their degrees of muscling - thickness of muscling in relation to skeletal size. Since the average thickness of backfat in relation to either carcass weight or length has been found to be a good indicator of the yield of the four lean cuts, these factors, together with a muscling evaluation, are used as the basis for the numbered grades.
Referring now to FIG. 1, there is shown where some of the standard measurements for average backfat thickness and carcass length are made. The average backfat thickness can be made perpendicular to the outside surface of the carcass opposite the first and last ribs and the last lumber vertebrae. Carcass length is generally measured from the anterior point of the aitch bone to the anterior edge of the first rib next to the backbone. Hot carcass weight is based on an entire carcass dressed packer-style--split into two sides down the back, jowles attached, and head, ham facings and leaf fat removed.
In some carcasses the actual average thickness of backfat is not representative of their degree of fatness. In such cases, the standards provide that an appropriate adjustment shall be made in the average thickness of backfat. In determining whether or not an adjustment is necessary, particular measurements are made in the backfat thickness at points other than those used in determining the average and the amount of fat in such areas as over the outside of the hams, directly anterior to the hip bone, over the edge of the loin, and at the juncture of the belly with the shoulder. For a carcass having a fat distribution indicative of a greater degree of overall fatness than that associated with its actual average thickness of backfat, the average thickness of backfat is then adjusted upward or downward as required and indicated by the measurements.
FIG. 2 illustrates the general relationships between average thickness of backfat, carcass length or weight, and grade for carcasses with muscling typical of their degrees of fatness. Within each grade, as carcass length or weight increases, average backfat thickness also increases. The rate of increase is typically one-tenth of an inch in average backfat thickness for a 3-inch increase in carcass length or about a 45-pound increase in carcass weight. For example, at a carcass length of 30 inches or a carcass weight of 165 pounds, the thickness of backfat at the juncture of the No. 1 and No. 2 grades is 1.4 inches. At the juncture of the No. 2 and No. 3 grades it is 1.7 inches and it is 2.0 inches at the juncture of the No. 3 and No. 4 grades. Also, as shown in FIG. 2, "thick" is the normal degree of muscling specified for carcasses at the juncture of the No. 1 and No. 2 grades; "moderately thick" muscling is specified for carcasses at the juncture of the No. 2 and No. 3 grades; and "slightly thin" muscling is considered normal for carcasses at the juncture of the No. 3 and No. 4 grades.
In view of the numerous measurements presently required and contemplated in the future to accurately determine the proper classification of carcasses, a means and method for making accurate and repetitive measurements on a carcass during processing is required. Bearing in mind that if one desires to grade and measure each hog during processing on a given day in a packing plant which can normally handle 4000 to 5000 carcasses per day, numerous measurements must be made and a means for accomplishing the measurements and recording the results in rapid order to properly classify individual carcasses and groups of carcasses.